<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>

<style>
    body {
        text-align: center;
    }
    
    .box {
        width: 1200px;
        height: 1210px;
        border: 1px solid red;
        margin: 0px auto;
    }
</style>

<body>
    <div class="box" id="box"></div>
</body>

<script>
    var box = document.getElementById("box");
    for (var i = 0; i < 100; i++) {
        for (var j = 0; j < 100; j++) {
            var b = document.createElement("div");
            b.style.border = "1px solid black";
            b.style.width = "10px";
            b.style.height = "10px";
            b.style.cssFloat = "left";
            box.appendChild(b);

            

            if (i % 2 == 0) {
                b.style.background = "#ff0000";
            } else {
                b.style.background = "#0000ff";
            }
        }
    }
</script>

</html>